The present invention relates to a torque distribution control apparatus for a four wheel drive.
In general, there has been known a torque distribution control apparatus for a four wheel drive which drives four wheels by engine outputs, wherein the torque distribution for each wheel is not always equivalent but is variably controlled so as to become optimum correspondingly to operating conditions.
For example, Japanese Unexamined Patent Publication No. 1-247223 has disclosed that the turning movement of a car is classified under three parts at the time of turning start, during turning and at the time of turning escape, so that the torque distribution is carried out correspondingly to the turning state of the car. In other words, the torque distribution on the rear wheel side is increased in order to enhance the small turn of the car. In addition, the torque distribution on the front wheel side is increased in order to enhance straightness at the time of turning escape.
On a low .mu. road of which friction coefficient (hereinafter referred to as .mu. if required) is small, a gain in momentum such as a lateral acceleration, a yaw rate and the like caused by drivers' steering is very smaller as compared with a high .mu. road. Accordingly, a quantity of drivers' steering is easily increased on the low .mu. road. In this case, steering wheels (front wheels) are driven with being excessively rolled. Consequently, the sufficient friction force cannot be obtained between the wheels and the road. Therefore, disadvantages are caused in running stability. In addition, it is hard for drivers to correct the change in movement of the car and steer the car.
On the other hand, there is proposed that a longitudinal torque distribution ratio is controlled such that the torque distribution for the front wheels is reduced and that for the rear wheels is increased. However, it is necessary to considerably increase the distribution ratio so as to obtain the required gain. In this case, the great burden is taken to the wheels (rear wheels) on which the larger driving torque is applied. Consequently, slip is easily caused so that the movement of the car changes sharply.
Furthermore, there is proposed that the torque distribution for right and left wheels is controlled correspondingly to a steering angle and a car speed. However, the low and high .mu. roads have different friction force, which is generated between the wheels and road, respectively. Consequently, it is hard to control the torque distribution uniformly.